As is known in the art, ink jet printers use one or more ink jet heads for projecting drops of ink onto a printing medium (such as paper) to generate text, graphical images or other indicia. Drops are projected from a minute external orifice in each head to the printing medium so as to form the text, graphical images or other indicia on the printing medium. A suitable control system synchronizes the generation of ink drops. It is important that the ink drops be of substantially uniform size, and also that the drops are applied consistently onto the printing medium so that printing is not distorted.
Existing ink jet technology, whether it is thermal jet or piezo-jet, can only jet micro-droplets with low viscosity liquids (typically 2-15 centistokes), such as water based inks, and only for short printing distances. In such existing ink jet technology, a pressure pulse is applied to a fluid chamber with sufficient pressure to overcome surface tension forces, thereby forming and ejecting a droplet of fluid from the ink jet nozzle. However, for jetting higher viscosity liquids (greater than 100 centistokes) with drop-on-demand requirement, there is no known ink jetting method.
In one basic type of ink jet head, ink drops are produced on demand, for example as disclosed in U.S. Pat. No. 4,106,032 issued to Miura, et al. on Aug. 8, 1978, the entire disclosure of which is herein incorporated by reference. In such drop-on-demand ink jet heads, ink in an ink chamber in the ink jet head, in response to a pressure wave generated from an electric pulse applied to a piezoelectric crystal, flows through an ink passageway in an ink chamber wall and forms an ink drop at an internal drop-forming orifice outlet located at the outer surface of the ink chamber wall. The ink drop passes from the drop-forming orifice outlet, through an air chamber, and toward a main external orifice of the ink jet head leading to the print medium. Continuous air under pressure is delivered to the air chamber and propels the ink drop through the air chamber and to the print medium.
However, such prior art drop-on-demand ink jet heads suffer from numerous disadvantages, drawbacks and/or limitations, for example as discussed in U.S. Pat. No. 4,613,875 issued to Le et al. on Sep. 23, 1986, and in U.S. Pat. No. 4,728,969 issued to Le et al. on Mar. 1, 1988, the entire disclosures of these patents are herein incorporated by reference. In an attempt to improve upon such prior art drop-on-demand ink jet heads, Le et al. discloses in the '875 patent an ink chamber with an ink drop-forming orifice outlet from which ink drops are generated in response to pressure waves caused by a piezoelectric crystal. This internal orifice outlet is centered in a projecting structure which extends toward an external orifice. The projecting structure is of a frustoconical or mesa-like shape. As stated therein, air flowing past the top (orifice outlet) of the projection prevents ink from wetting anything but the top of the projection, resulting in highly uniform ink drop formation with a single uniform dot being produced on the printing medium in response to each pressure wave.
Reproduced herein as FIG. 1, for the purpose of illustration, is the prior art FIG. 2 from the '875 patent to Le et al. showing this projecting structure of Le et al. As can be seen therein, the projection extends a length “D” into an annular air chamber, almost completely to the external orifice, with only a small spacing “E” there between. Le et al. discloses that the length of the projection is in the range of 50-90 μm with a preferred distance of 60 μm.
However, this configuration suffers from numerous disadvantages, drawbacks and/or limitations itself. For example, Le et al. uses continuous air flow to accelerate the ink drop. As such, if the velocity is too high, the continuous air flow will adversely affect the ink drop as it is propelled to the printing medium, resulting in a poor or failed printing result. If the velocity is too low, then the ink drop will not properly form and will not be propelled at a high enough velocity, again resulting in a poor or failed printing result. These limitations are particularly apparent with higher viscosity liquids.
Therefore, a need exists for an improved air assisted drop-on-demand ink jet head which is directed toward overcoming these and other disadvantages of prior art devices. Accordingly, to address the above stated issues, a method and system for jetting high viscosity liquids to form micro-droplets and at high velocity for achieving increased print distances is needed. The exemplary teachings herein fulfill such a need. It is desired that the methods and systems for providing the above benefits be applicable to any instances or applications wherein micro-droplets of high viscosity liquid are to be dispensed.